1. Technical Field of the Invention
The present invention relates to the air interface of a cellular telephone network and, in particular, to an air interface of a cellular telephone network supporting multiple modulation levels for communications.
2. Description of Related Art
The TIA/EIA Interim Standard IS-136 specified air interface of the digital advanced mobile phone service (D-AMPS) system separates the allocated cellular frequency spectrum into a plurality of thirty-kilohertz channels. Each channel is divided into 6.67 millisecond (ms) time slots, with three consecutive time slots forming a time division multiple access (TDMA) block. The modulation scheme used is differential quadrature phase shift keying (DQPSK), a relatively low-level modulation (LLM), with onehundred sixty two symbols (of two bits each) per time slot.
Two types of channels are defined for the conventional air interface: the digital control channel (DCCH) and the digital traffic channel (DTC). The digital control channel is a multi-user channel that is used for controls and services such as registration, authentication, call setup, and the like. The digital traffic channel is a circuit switched single user channel that is assigned at call setup and handoff, and is used to handle a voice and/or data communication between users of the cellular system and users in a fixed or other cellular system. The D-AMPS standard supports full-rate, double-rate and triple-rate digital traffic channels for user data communications using one, two and three time slots per block, respectively,.
On the downlink over the D-AMPS air interface, every time slot, whether used for a digital control channel or a digital traffic channel, carries one-hundred thirty symbols of user information. This equates to a transfer of two-hundred sixty bits of user information every twenty milliseconds. Thus, each slot supports a communications rate of thirteen kilobits per second (kb/s). In practice, however, the actual information transfer rate is much less due to the inclusion of error protection bits. For example, voice traffic is transmitted over a full-rate digital traffic channel using approximately five kb/s of error protection for the approximately eight kb/s of digitized speech provided by a vocoder. For data traffic, on the other hand, the transmission over a full-rate thirteen kb/s digital traffic channel is made with a corresponding data rate of 9.6 kb/s.
The permitted data rates for voice and data communications over the digital traffic channel may be increased if double-rate or triple-rate traffic channels are used. The main difficulty or drawback with the use of multi-slot (i.e., multi-rate) operation is that the mobile stations which are being used for the communications utilize idle digital traffic channel time slots, where no communications over the air interface with the base station are being made, to make mobile assisted handoff (MAHO) measurements of the received signal strength from neighboring base stations. When configured for triple-rate voice or data communications, the mobile station is in essence communicating continuously, which leaves no time for making signal strength measurements. Frame stealing, wherein the mobile station interrupts communication for one or more time slots to make signal strength measurements, has been proposed as a possible solution to support multi-slot, and in particular triple-rate, communications. This is not a preferred solution as some communications data loss or interruption in communications continuity may occur.
A modification of the D-AMPS system has been proposed (referred to as D-AMPS+) which would enable higher rate communications without the need for multi-slot operation. For voice communications, a high-rate vocoder is used to provide higher quality digitized speech, and a high-level modulation (HLM) scheme providing more bits per transmitted symbol, such as sixteen level quadrature amplitude modulation (16-QAM), is then implemented on the digital traffic channel to increase the payload capable of being carried in each time slot. The use of high-level modulation is complementary to multi-slot operation to achieve the highest capacity in a radio channel of a given bandwidth. High-level modulation is thus preferred for a number of reasons. First, it preserves network capacity. Second, it minimizes power consumption in the mobile station, resulting in a longer talk time. Third, it facilitates conventional mobile station operation in making MAHO signal strength measurements during idle time slots. In summary, D-AMPS+ maintains the same air interface slot structure as in D-AMPS, thus insuring backward compatibility, while simultaneously providing for higher throughput due to its support of a high-level modulation scheme.
An enhancement of the D-AMPS system has also been proposed which would facilitate the support of packet data communications over the air interface and compatibility with the cellular digital packet data (CDPD) network. In the enhanced D-AMPS system, two new types of packet data channels are provided. The first is a packet control channel (PCCH) which comprises a multi-user channel much like the previously described digital control channel (DCCH), and also used for controls and services such as registration, authentication, call setup, and the like, as well as for the transmission of data packets. The second is a packet traffic channel (PTCH) comprising a single user channel much like the previously described digital traffic channel (DTC), again assigned at call setup, and used to handle a packet data communication between users. The structure of these channels is very much like that of the D-AMPS channels, and the channels utilize the relatively low-level differential quadrature phase shift keying (DQPSK) modulation scheme. Again, multi-slot (up to three slots for triple-rate) operation of the channels is supported providing a maximum aggregate user payload of approximately thirty kb/s. Similar drawbacks as discussed above with respect to digital traffic channel multi-rate operation are encountered with multi-rate packet traffic channel operation.
The high level modulation also applies to packet channels (traffic or control). Mobile stations capable of operation using only the low-level modulation scheme (i.e., enhanced D-AMPS only mobiles) are assigned to use the low-level modulation packet control channel and packet/digital traffic channels. D-AMPS+ mobile stations, on the other hand, may be assigned the high-level or low-level modulation packet control channel and packet/digital traffic channels depending on channel conditions (such as interference, bit error rate, word error rate, fading rate and the like).
For such D-AMPS+ mobile stations, a mechanism is needed for effectuating the selection of and a transition of operation between the low-level modulation and high-level modulation on the digital traffic channels and the packet traffic channels. The present invention provides such a mechanism.